Vapor generators



Feb. 24, 1970 RA. GRAMS.

VAPOR GENERATORS 2 Sheets-Sheet 1 Filed Feb. 27, 19 68 m T m m RichardA.Grams 2 ATZRNEY Feb. 24, 1970 R. A. GRAMS VAPOR GENERATORS 2Sheets-Sheet 2 Filed Feb. 27. 1968 r||||||||l|ll II Ik United StatesPatent O U.S. Cl. 122-478 7 Claims ABSTRACT OF THE DISCLOSURE Animproved marine vapor generator of the type which includes a reheaterand the means for by-passing the combustion gases around the reheaterwhen vapor flow through it is stopped as in the case of maneuvering andastern operation of the ship. Improvements to the generator include afirst convection gas pass, located between the superheater and reheaterpasses, arranged with sufficient saturated surface to reduce andmaintain the reheater pass gas temperatures within safe operating limitsand controlling reheat steam temperature by regulating the flow ofcombustion gases through both the bypass portion of the reheater passand the bypass portion of the first convection gas pass. A major portionof the economizer is exposed to the entire flow of combustion gasesthereby achieving stable, efficient operation during all operatingmodes.

The invention relates to a ship propulsion system and more particularlyto marine type vapor generators equipped with a reheater and to themeans for controlling the flow of combustion gases over the saidreheater.

The reheating of steam between turbine stages is well recognized as adesirable step in obtaining the high efficiencies required of todayspower plant systems, however the application of the reheat principle toa ships propulsion system introduces certain factors which are notnormally encountered in stationary instalations. One such factor is thepractice of marine systems to exclude reheat and part of the steam cycleduring astern operation, maneuvering and inport operation of the ship.

Heat exchanger surface intended for the superheating or reheating ofsteam is normally located in a zone where operating gas temperatures arefar above recognized design temperature use limits for metals, however,such use is made possible by the metal cooling effect derived from steampassing through the tubes of these heat exchangers. A problem arisesduring astern operation, maneuvering and inport operation when no steamis passed through the reheater, thereby producing a condition where tubemetal temperatures will exceed safe design limits unless remedialmeasures are taken. One manner of overcoming this problem has been tolocate the reheater in one of two parallel gas passes and to providedampers for preventing the flow of hot combustion gases over thereheater when the latter is not in use, as is shown in US. Patent No.3,280,559.

In the present state of the art, it is costly and often impractical toprovide a gas-tight damper to contain and control a hot and corrosivefluid such as boiler flue gas. An object of the present invention is toprovide protection for a tubular reheater by disposing sufiicient heatexchange surface ahead of the reheater to lower the gas temperature toacceptably safe limits before it enters the reheater. This heat exchangesurface may comprise saturated wall tubes lining a first convection gaspass located between the superheater and reheater gas passes. Animportant feature of the saturated surface arrangement is the formationof an open pass cavity. Heat transfer in the cavity is by means ofradiation and is, there fore, primarily a function of gas temperatureentering the cavity rather than a function of gas mass flow as isconvective heat transfer. Thus for a given boiler rating the cavity heatabsorption will be approximately the same regardless of whether or notthe reheater is being bypassed. This in turn means that the cavity willbe much more effective in reducing gas temperature during reheaterby-pass operation as the gas flow to the reheater is reduced to minimumvalues (that corresponding to damper leakage). In this manner thereheater can be protected from overheat during by-pass operation andwill also have some extra thermal potential available in the gas fornormal operation.

Another object of the invention is to facilitate the control ofsuperheated steam temperature through the use of saturated surfacerather than superheater surface to modulate the gas temperature tothereby protect the reheater. It can be readily seen that if steamgenerating apparatus of the general type disclosed by applicant shouldhave a portion of the superheater surface located in the reheater gaspass that portion of the superheater would remain virtually idle duringastern operation, thereby effectively reducing the amount of superheatersurface area exposed to the flow of combustion gases. Since superheatersurface requirements are basically dependent on fuel input it would meanthat, for equal firing rates, the effective superheater surface andtotal absorption will differ substantially for astern and forwardoperation of the vessel. Obviously this would impose a most difficultoperating burden on the system for controlling superheated steamtemperature over the units load range.

Still another object of the invention is to achieve efiicient, stableoperation of the unit throughout all operating modes by exposing themain portion of the economizer heat surface to the entire flow ofcombustion gases. This main portion may comprise upwards of percent ofthe total economizer heating surface nad may be extended surface typetubes for added heat transfer capability. The remaining portion ofeconomizer surface may be bare tubes located in the alternate gaspassage provided for bypassing the reheater, its primary function beingto lower the fiue gas temperature below say 1000 F. to protect thedampers in the gas flow path. It can be readily seen that with thisarrangement of economizer approximately the same feedwater temperaturewill be available at the boiler drum for specific firing ratesregardless of the mode of operation.

A further object of the invention is the control of reheat steamtemperature by regulating the gas mass flow across the reheater. This isaccomplished through the selective positioning of dampers located in thealternate or by-pass gas outlet passage of the first convection gaspass. These dampers are designed to effectively control the quantity ofgas flowing in the by-pass. On the other hand, it is contemplated thatthe dampers at the outlet of the reheater gas pass will operate eitherin a fully open or closed position, their sole function being to inhibitthe flow of gases across the reheater during astern operation,maneuvering or inport operation. These dampers will necessarily be ofthe type which provide closure means rather than positive controlcharacteristics.

In the drawings:

FIGURE 1 is a diagrammatic sectional side view along lines 1-1 of FIGURE2 of an improved vapor generator for use in a ship propulsion system.

FIGURE 2 is a diagrammatic sectional plan view along lines 22 of FIGURE1.

Referring to FIGURES 1 and 2 there is shown a boiler setting 10comprising front and rear walls 11 and 12 and sides walls 13 and 14. Theboiler setting 10 is enclosed by casing 15 and insulation 16. Thesetting 10 is partitione'd into a furnace chamber 17, a superheater gaspass 18, a first convection gas pass 19, a reheater gas pass 20, and asecond convection gas pass 21. Furnace chamber 17 is defined by sidewall 13 and sections of front wall and rear walls 11 and 12 and apartition wall 22. The walls enclosing furnace chamber 17 are of tangenttube to tube construction with the exception of a portion of partitionwall 22 which includes a staggered tube intermediate section forming ascreened inlet 23 to the superheater gas pass 18, and the section offront wall 11 which includes the openings 24 to accommodate the fuelburners (not shown). The tubes making up the furnace floor 34, partitionwall 22 and side wall 13 originate in lower header 25 and terminate insteam drum 26. The tubes in the furnace section of front wall 11originate at lower header 27 and terminate in upper headers 28 and 29which are connected to steam drum 26 through riser tubes 30. The tubecircuitry of the furnace section of the rear wall 12 (not shown) isidentical to that of the front wall 11 with the exception that there areno burner openings.

The superheater gas pass 18 contains the entire superheater heatexchange surface composed of upright generally U-shaped tubes 35 spacedacross the whole width of the gas pass and comprising a secondarysuperheater 36, a tertiary superheater 37 and a primary superheater 38serially arranged in that order in the direction of gas flow. Thesections of front and rear walls 11 and 12 which define the superheatergas pass are of wide spaced construction with fiat studs 39 filling-inthe tube spacing. The tubes in the superheater gas pass section of frontwall 11 originate at lower header 40 and terminate in steam drum 26. Thetube circuitry of the superheater gas pass section of the rear wall 12(not shown) is identical to that of the front wall 11 with the exceptionthat there are access doors 41 to allow entry into the lanes betweensuperheater banks. The superheater gas pass 18 has a refractory linedsloped floor 42. Floor 42 also serves as the roof of header vestibuleenclosure 43. The first convection gas pass 19 is situated downstreamgas flow-wise of superheater gas pass 18. Gas passes 18 and 19 areseparated by partition wall 44 which includes a staggered tubeintermediate section forming a screened gas inlet 45 to first convectiongas pass 19. Superheater gas pass 18 includes transverse screen tubewall 46 used primarily to support the tertiary superheater 37 throughbrackets 47. Primary and secondary superheaters 38 and 36 are supportedby partition walls 44 and 22 respectively through similar brackets.

First convection gas'pass 19 is a vertically elongated pass disposedintermediate and laterally adjoining superheater gas pass 18 andreheater gas pass 20 and defined by sections of front and rear tubewalls 11 and 12, partition walls 44 and 48. Partition wall 48 includes astaggered tube lower section forming a screened inlet 50 to heater gaspass 20, two staggered tube intermediate sections forming tube screens51 and 52 located adjacent gas inlet 45 in partition wall 44. Aneconomizer 49 is placed between screens 51 and -2 and comprises sinuoustubes 76 horizontally disposed across the first convection gas pass 19with the tube ends attaching to headers 60 and -61 respectively. Thetubes of partition walls 44 and 48 originate in lower drum 53 andterminate in steam drum 26. The tubes forming the first convection gaspass section of front wall 11 originate in lower header 54 and terminatein upper header 55. The tube circuitry of the first convection gas passsection of the rear wall 12 (not shown) is identical to that of thefront wall 11. The top outlet section 32 of first convection gas pass 19situated above header 55 is defined by casing and includes a damper 56adjacent the inlet 57 of second convection gas pass 21. Partition wall44 has an access door 59 to header vestibule 43.

Reheater gas pass is a vertically elongated pass adjacently parallel tofirst convection gas pass 19 and d fin d. by part tion wa l 48, side wal14 a d s c on of front and rear walls 11 and 12. Reheater gas pass 20includes a reheater 62 comprising sinuous tubes 75 forming a pluralityof separate banks and disposed across the width of the gas passage withtheir ends attaching to headers 63 and 64 respectively. The tubes in thereheater gas pass section of front wall 11 originate at lower header 65and terminate at upper header 55. The tube circuitry of the reheater gaspass section of the rear wall 12 (not shown) is identical to that of thefront wall 11. The outlet section 33 of reheater gas pass 20 situatedabove header 55 is defined by casing 15 and includes a damper 66adjacent the inlet 57 of second convection gas passage 21.

Second convection gas pass 21 is a vertically elongated gas pass locatedabove and in communication with the outlets of both the first convectiongas pass and the reheater gas pass through dampers 56 and 66respectively. Second convection gas pass 21 is defined by walls formedof casing 15 and includes a main economizer 67 comprising sinuous tubes74 forming a plurality of separate banks and disposed across the widthof the gas pass with the tube ends opening into headers 68 and 69.

Downcomers 70 connect the steam drum to all of the furnace wall supplyheaders, for example, headers 25 and 27 and to lower drum 53.Additionally supply tubes 71 connect lower drum '53 to front wallheaders 40, 54 and 65 and also to similarly disposed headers associatedwith the rear wall. Steam drum 26 includes steam-water separators 72.For the sake of clarity the connecting steam piping between drum 26 andthe various superheater headers 73 is not shown.

In normal ahead or forward operation of the ship the steam-water fluidflow path of the steam generating unit is as follows: A controlledquantity of feedwater is admitted to inlet header 68 of main economizer67 and is heated by combustion gases while passing through sinuous tubes74 to outlet header 69. The heated feedwater is then piped (not shown)to header 60 of by-pass 49 and is further heated by combustion gaseswhile passing through sinuous tubes 76 to outlet header 61. An alternatearrangement has the heated feedwater from main economizer 67 enteringheader 61 of by-pass economizer 49 and leaving through header 60. Afterpassing through by-pass economizer 49 the heated feedwater is introducedby a conventional feed pipe (not shown) into steam drum 26 to maintain apredetermined water level. A natural circulation cycle takes placewhereby water from steam drum 26 is circulated through downcomers 70 tolower drum 53 to supply headers 25 and 27 etc. and from the lower drumthrough supply tubes 71 to supply headers 40, 54 and 65 as well assimilarly disposed headers associated with the rear wall. The waterleaving the above mentioned lower drum and supply headers is heatedbecoming a steam-water mixture during its passage upwardly through thetubes of front and rear walls 11 and 12, side walls 13 and 14 andpartition walls 22, 44 and 48 and screen wall 46. The steam-watermixture is returned to steam drum 26 directly from the partition wallsand sidewalls and by way of discharge headers 28, 29 and 55 via risertubes 30. The steam-water mixture upon entering the steam drum 26 passesthrough separators 72. The water fraction leaving separators 72 isreturned to the water space of drum 26 from whence it enters downcomers70 to repeat the circulating flow cycle. The saturated steam leavingseparators 72 is passed through primary scrubbers 77 and secondaryscrubbers 78 and after leaving steam drum 26 is conveyed throughappropriate piping (not'shown) to be passed serially through theprimary,

secondary and tertiary superheaters 38, 36 and 37. These superheatersections are arranged for series flow and may include an attemperator(not shown) between the outlet of the primary superheater 38 and theinlet to the secondary superheater 36 for steam temperature control. Thesuperheated steam upon leaving tertiary superheater 37 is introducedinto the high pressure turbine (not 5 shown). Steam leaving the highpressure turbine is conveyed through appropriate piping (not shown) toreheater inlet header 63 to be reheated during passage through tubes 75and is discharged from reheater outlet header 64 to be conveyed to thelow pressure reheat turbine through appropriate piping (not shown).

The combustion gas path is as follows: Fuel and combustion air areintroduced through burner openings 24, and after transferring a portionof their heat content to the water cooling tubes lining furnaceenclosure 17 the gases are discharged through opening 23 intosuperheater gas pass 18 where heat from the gases is transferred to thesuperheater and saturated wall enclosure surfaces. From passage 18 thegases are discharged through opening 45 into first convection gas pass19 where the flow of gases may be divided into two flow paths, one beingdirected downwardly toward opening 50 with heat being transferred to aportion of the water cooled enclosure walls of passage 19. The other gasflow path is directed up wardly across screen tubes 51, 52 and 31, andeconomizer 49 toward outlet 32 and damper 56 with heat being transferredto these heat absorbing surfaces and to adjacent portions of thesaturated Walls of passage 19. The gases discharging into reheater gaspass 20 are directed upwardly across reheater 62 and screen tubes 31through outlet 33 toward damper 66 with heat being transferred toreheater 62 and the saturated water wall enclosure surfaces. Wheneversteam is being passed through reheater 20, damper 66 will be in wideopen position. Reheater outlet steam temperature is controlled bymodulating damper 56 thereby varying the gas mass flow being directed togas pass 20 and across reheater 63. A closing of damper 56 will forcemore gases through inlet opening 50 and into gas passage 20 resulting ina rise in reheat steam temperature for equivalent steam flows, and theconverse effects will result when the quantity of gas flowing isreduced. The gases emerging from gas passage 32 and 33 form a singleflow path through second convection gas pass 21 wherein heat istransferred to the feedwater flowing through economizer 67.

During normal astern operation of the ship the steamwater cycle differsfrom normal ahead operation in that the steam upon leaving tertiarysuperheater 37 is directed to a high pressure condensing type turbine,(not shown) normally referred to as an astern turbine, and whosedirection of rotation is opposite from that of the turbine used forahead operation. Since for this service there customarily is no fiowthrough the reheater, the steam leaving the astern turbine is condensedand returned to the condensate system (not shown) for use as feedwaterfor the boiler.

The combustion gas path for astern operation differs from that of aheadoperation in that substantially all of the gases leaving superheater gaspass 18 are directed upwardly through first convection gas pass 19 todamper 56 and on through second convection gas pass 21. With this modeof operation, i.e. no steam flow through the re heater 62, damper 66will be closed and damper 56 wide open, thereby substantially stoppingthe flow of combustion gases into and through reheater gas pass 20.

Since the medium to be controlled is a hot corrosive gas of up to 1000F., it is economically unfeasible to construct a commerciallyserviceable gas tight shut-off damper. Accordingly as a practicalconsideration damper 66 is designed for an expected leakage of upwardsof 10 percent of the total gas flow when in the closed position. Similarconsiderations respecting damper 56 resulted in the first convection gaspass 19 being arranged with sufficient heat absorbing saturated Wallsurface to reduce the gas temperature entering gas passage 20 to below1000 F. assuming leakage of upwards of 20 percent of the total gas flow.

Thus for all normal operating conditions the gas temperature at dampers56 and '66 will remain within safe allowable operating limits for dampermetal temperature.

A particular example of a marine reheat vapor generator incorporatingthe present invention could operate at the conditions set forth in TableI.

TABLE I Normal Rate:

superheated steam flow, lb./l1r 166, 200 210,000

Reheated steam flow, lb./hr 162, 800 Steam temperature:

superheater outlet temp, F 1, 000 1,000 Reheater outlet temp, F 950 Gasweights:

Total gas. lb./hr 238, 300 255, 820 Gas across superheate 238, 300 255,820 Gas across reheater. 190, 700 51, 200 Gas by-passing reheater 47,600 204, 620 Gas temperature:

Leaving furnace, F 2, 350 2, 380 Entering bypass economizer 1,070 1,135Leaving by-pass economizer.-. 695 880 Entering by-pass dampers" 685 850Leaving reheater 710 970 Entering reheater dampers 705 950 Entering maineconomizer 700 860 Leaving main economizer 320 335 Feedwatertemperzture:

Entering economizer, F 280 28) Leaving main economizer 420 475 Leavinglay-pass economizer 450 535 What is claimed is:

1. In a ship propulsion system wherein superheated and reheated steam isselectively supplied to a forward drive turbine and only superheatedsteam to an astern drive turbine, the combination with the propulsionsystem of a steam generating superheating and reheating unit comprisingwalls including steam generating tubes forming a setting, partition wallmeans including steam generating tubes dividing the setting into afurnace, a superheater gas pass laterally adjoining the furnace andopening at its inflow end to the furnace across the full width and alongthe major portion of the height thereof, a reheater gas pass, and avertically elongated first convection gas pass disposed intermediate andlaterally adjoining the superheater and reheater gas passes and arrangedto receive all the superheater gas pass outflow, said first gas passhaving one gas outflow portion opening to the lower end of the reheatergas pass and another gas outflow portion for by-passing gases around thereheater gas pass,

means for firing the furnace,

a bank of vertically arranged return bend superheater tubes in thesupeheater gas pass,

an economizer in said other portion of the first convection gas pass,

a bank of reheater tubes in the reheater gas pass and constituting theentire reheater heating surface,

a second convection gas pass arranged to receive gas outflow directlyfrom the reheater gas pass and from said other portion of the firstconvection gas pass,

another economizer in the second convection gas pass connected for flowof fluid with the first named economizer and constituting a majorportion of the total economizer heating surface, and

damper means for regulating gas outflow from said other portion of thefirst convection gas pass and from the reheater gas pass.

2. A ship propulsion system according to claim 1 wherein the dampermeans includes a first set of dampers at the outlet of the reheater gaspass and a second set of dampers at the outlet of said upper portion ofthe first convection gas pass.

3. A ship propulsion system according to claim 2 wherein the first setof dampers is fully open and the second set of dampers is used toproportion the gas flow between the reheater gas pass and the firstconvection gas pass when reheated steam is supplied to said forwarddrive turbine.

4. A ship propulsion system according to claim 2 wherein the first setof dampers is fully closed and the second 7 set of dampers is fully openwhere only superheated steam is supplied to said astern drive turbine.

5. A ship propulsion system according to claim 1 wherein the othereconomizer is connected for series flow of fluid to the first namedeconomizer 6. A ship propulsion system according to claim 5 whereinfluid flow through the first named economizer is in indirect parallelflow heat absorbing relation with the gases.

7. A ship propulsion system according to claim 5 wherein fluid flowthrough the first named economizer is in direct parallel flow heatabsorbing relation with the gases.

References Cited UNITED STATES PATENTS 5/1947 Boland 122478 12/1958Hutchings et a1. 122480 10/1961 Hamilton et al. 122478 12/1967 Signell122-480 KENNETH W. SPRAGUE, Primary Examiner US. Cl. X.R.

